Published: October 29, 2010

BAOTOU, China — When Japanese mineral traders learned in late September that China was blocking shipments of a vital commodity, the word came not from a government announcement but from dock workers in Shanghai.

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Doug Kanter for The New York Times

In Baotou, a smoggy city in China’s Inner Mongolia, the air this week has an acrid, faintly metallic taste. Half of the global supply of rare earths comes from the hills north of the town.

And on Thursday, the traders began hearing that the unannounced embargo of so-called rare earthminerals was ending — again, not from any Chinese government communiqué, but though back-channel word from their distributors.

Throughout the five weeks of the embargo, even when China expanded the rare earth shipping halt to include the United States and Europe, Beijing denied there was a ban.

Whatever it was called, a shipping suspension that started amid China’s diplomatic dispute with Japan over a wayward fishing trawler escalated into a broader international trade issue.

The episode alarmed companies around the world that depend on rare earths, minerals that help make a wide range of high-tech products, including smartphones and smart bombs. China currently controls almost all of the world’s supply of rare earths, for which demand is soaring.

To many outsiders, the undeclared embargo looked like a pure power play — a sign China would wield its growing economic might and apply its chokehold on an important industrial resource with little regard for the conventions of international trade. The export quotas China continues to impose on rare earths, even when it does let ships leave the docks, are restricting global supplies and causing world market prices to soar far beyond what Chinese companies pay.

From the Chinese perspective, though, the issue looks very different.

China feels entitled to call the shots because of a brutally simple environmental reckoning: It currently controls most of the globe’s rare earths supply not just because of geologic good fortune, although there is some of that, but because the country has been willing to do dirty, toxic and often radioactive work that the rest of the world has long shunned.

Despite producing 95 percent of the world’s rare earths, China has only 37 percent of the world’s proven reserves. Sizable deposits are known to exist in the United States, Canada, Australia, India and Brazil, among other places.

Many of those countries, responding to the rising demand for rare earths and alarmed by the recent embargo, are now scrambling to develop new mines or renovate ones long considered not to be worth the effort. That includes an abandoned mine in California that the American company Molycorp is trying to refurbish.

But experts say that any meaningful new production from outside China is at least five years away, and that it will come with its own environmental cost calculus.

“China’s rare earth output cannot be raised fast enough to meet the entire world’s needs, as there are environmental factors to be taken into consideration with an increase in rare earth production,” said Zhang Peichen, the deputy director of the government-backed Baotou Research Institute of Rare Earths, the main research group for the Chinese industry.

Across China, rare earth mines have scarred valleys by stripping topsoil and pumping thousands of gallons of acid into streambeds. The environmental costs are palpable here in Baotou, a smoggy mining and steel city in China’s Inner Mongolia, where the air this week had an acrid, faintly metallic taste.

Half of the global supply of rare earths comes from a single iron ore mine in the hills north of Baotou. After the iron is removed, the ore is processed at weather-beaten refineries in Baotou’s western outskirts to extract the rare earths minerals.

The refineries and the iron ore processing mill pump their waste into an artificial lake here. The reservoir, four square miles and surrounded by an earthen embankment four stories high, holds a dark gray, slightly radioactive sludge laced with toxic chemical compounds.

The deadly lake is not far from the Yellow River watershed that supplies drinking water to much of northern China. The reservoir covers an area 100 times the size of the alumina factory waste pond that collapsed this month in Hungary, inundating villages there and killing at least nine people.